Gas discharge apparatus



p 1963 R. CARRUTHERS ETAL 3,105,027

GAS DISCHARGE APPARATUS Filed Oct. 8, 1958 2 Sheets-Sheet 1 INVENTORS ROBERT CARRUTHERS PETER CLIVE THONEMANN 'Aftbrneys Sept. 24, 1963 R. CARRUTHERS ETAL 3, 05,027

GAS oxscmmcs APPARATUS Filed Oct. 8, 1958 2 Sheets-Sheet 2 E r\ 's "u "3 INVENTORS ROBERT CARRUTHERS PETER CLIVE THONEMANN Attorneys United States Patent Ofifice 3,1fifi27 Patented Sept. 24, 1963 3,105,027 GAS DISCHARGE APPARATUS Robert Carruthers, Abingdon, and Peter Clive Tlionemann, Springfield, England, assignors to United Kingdom Atomic Energy Authority, London, England Filed Oct. 8, 1958, Ser. No. 766,124 Claims priority, application Great Britain Oct. 11, 1957 2 Claims. (Cl. 204-1932) This invention relates to gas discharge apparatus and I38 one application in apparatus of the type in which a ugh-current ring discharge is induced in a gas contained n a torus, the discharge forming the single-turn secndary winding of a pulse transformer. Apparatus of his kind, for use in research into the production of conrollcd thermonuclear reactions is described in the specilcation of co-pending application No. 692,500, filed )ctober 25, 1957, now Patent 3,054,742.

The electrical circuit of the apparatus described in the pecification of the aforementioned application consists ssentially of a capacitor which is charged from an exernal source and then discharged through the primary vinding of the transformer. When the voltage across he capacitor starts to reverse, an ignitron connected cross the winding is fired and both the primary and the lischarge currents die away exponentially from their eak values. The object of firing the ignitron at this ime is to prevent the voltage on the capacitor reversing.

The theory of operation of the above-described appaatus is that thermonuclear fusion reactions will take lace between ions of the gas in the torus as a result f these ions being heated by the collisions they make rith electrons in the ring discharge. However a finite true is required for the ions and electrons to come into iermal equilibrium and, under some conditions, the type f circuit described above may not maintain the disharge current at a suificiently high value for a sufficiently mg time for this equilibrium to be achieved.

It is an object of the present invention to provide means nabling a high value of discharge current to be main lined for a prolonged period.

According to the present invention a gas discharge aparatus wherein a high-current ring discharge is induced 1 gas in a vessel by the application of a short higholtage pulse from a first energising circuit to a primary 'inding of a pulse transformer whereof said ring disharge forms a single-turn secondary winding is charcterised in that there is provided a second energising ircuit adapted to produce a relatively low voltage outut of relatively long duration and means for coupling aid second energising circuit to said secondary winding 1 order to maintain the discharge current at a high alue for a prolonged period when the high voltage pulse as fallen to zero.

Said first energising circuit may comprise a capacitor 1d switch means for discharging said capacitor through re primary winding of said pulse transformer.

Said second energising circuit may comprise a changed :lay line and may be adapted to store energy at high )ltage, a second pulse transformer being provided for educing said high voltage to the required low voltage. The present invention takes advantage of the fact that ice the discharge is established its impedance is largely :sistive and of very low value, so that a relatively small voltage provides a primary current sufiicient to maintain the discharge current at a high value.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein:

FIG. I is a semi-schematic circuit diagram of one embodiment of the invention as applied to an experimental gas discharge apparatus of the type described in the specification of the above-mentioned copending application.

FIG. 2 shows waveforms in the circuit of FIG. 1.

Referring to FIG. 1, a gas discharge apparatus of the kind set forth is shown as comprising a metal torus 1 containing gas (deuterium or a deuterium/tritium mixture) in which a pinched ring discharge is induced by a transformer T1. The torus 1 is provided with. a toroidal winding L11 through which a current is fed from a source 2 to establish an axial magnetic field for reducing spatial instabilities of the discharge, as explained in the aforementioned specification. The transformer T1 is provided with a bias winding L10 fed from a source 4 through a choke L12, as is also described in the aforementioned specification.

The primary winding of transformer T1 is divided into five separate sub-windings L8, L9, L13, L14 and L15 fed in parallel from five identical cncrgising circuits. Only one of these five circuits is shown in detail, viz. the circuit connected to winding L8. The other four circuits are shown schematically as blocks 11, 12, 13 and 14 connected respectively to windings L9, L13, L14 and L15.

The circuit shown in detail comprises a capacitor C6 of l820 f., 24 kv. working, which is chargeable through a resistor R6 of 250 ohms and a Jennings high vacuum switch S8 from a source 3. Source 3 comprises a sixphase transformer and rectifier, the input to the transformer being servo-controlled by an induction regulator so that the source delivers a constant charging circuit of 1a. until capacitor C6 is fully charged to 24kv., and thereafter a trickle charge to maintain the voltage. Across C6 is connected a Jennings switch S10 in series with a resistor R8 of 250 ohms as a safety shorting switch. Capacitor C6 is discharged through winding L8 by firing a series ignitron 11 (BTH type BKl94). A second ignitron l2 (BTH type BK194) is connected between the cathode of 11 and the earth point of the circult, and a capacitor C7 of 2 d. and a mechanical switch S7 are connected between the anode of II and the earthy side of the circuit. The switch S7 is of the high-speed compressed-air operated type, with an operating time of about 57 milliseconds and contacts capable of carrying about ka. A protective series resistor R7 of 0.08m is included in the discharge circuit to protect C6 by damping any oscillations resulting from the accidental breakdown of l2.

Return connections from the five primary windings on T1 are taken to the earthy sides of the five circuits through the five secondary windings L6, L16, L17, L18 and L19 of a pulse transformer T2, having like T1 a bias winding L7 energised from a source 5 through a resistor Rlt) in order to achieve the maximum flux swing. The primary winding L5 of T2 is connected through a mechanical switch S6, similar to S7, to a delay line comprising four inductors l-1L4 and five capacitors C1-C5 each of 1820 f, 24 RV. working. Across these capacitors are connected safety shorting switches 51-55 of the Jennings type, in series with resistors Rl-RS of 250 ohms. The leakage inductance of the primary winding L constitutes a fifth inductor in the delay line. The delay line capacitors C1C5 are charged from a source 18, similar to source 3, through a Jennings switch S9 and a resistor R9 or" 250 ohms. Ignitrons I3 and I4 are connected across C1 and C5 respectively. The circuit connected to the primary winding of T2 constitutes a second energising circuit.

Transformer T2 has tappings providing step down ratios of 6/12/24/36/48/72/96/144/192:1. Inductors L1L4 have tappings enabling their inductances to be varied between 5 and 320 rnh. The five primary windings on T1 are distributed windings wound on the surface of the torus and can be connected in three series-parallel arrangements to give step-down ratios of 6/ 12/ 24:1.

The operation of the circuit of FIG. 1 is shown in FIG. 2, which displays waveforms of the voltage V across the five primary windings and the current I in these windings. FIG. 2 is not drawn to scale. Assuming the capacitors C6 in each of the five circuits and also the capacitors C1-C5 in the delay line circuit to be charged, all switches to be open and all ignitrons unfired, the switch S6 is closed at a time :1 and the transformer T2 is energised. Because of the step-down ratio of T2, a relatively small voltage is thus made available across the secondary winding L6 which is not however applied to the primary windings since I1, 12 and S7 are open-circuit. At a time 12, about 2 milliseconds after t1, I1 is fired and the full voltage on C6, plus the voltage across L6, is applied across L8, L9 etc. The current in each winding, and the discharge current in the torus, rise rapidly, the voltage across C6 starts to fall and the delay line starts to discharge.

Most of the energy initially stored in C6 is used to build up the discharge in the torus, and becomes stored in the magnetic field of the discharge. Once the discharge has built up, its reflected impedance is largely resistive and of very low value, so that quite a small voltage across the primary windings sufiicies to provide a primary current adequate to maintain the discharge current at its peak value. At time t3, when the current I has reached substantially its peak value and the voltage across C6 has fallen substantially to zero, ignitron I2 is fired, shorting out C6. The prevents the voltage across C6 from reversing, with consequent damage to the capacitor, but leaves applied across L8 the voltage across L6 which, for the reasons explained above, is now sufiicient to maintain the discharge current. Since I1 and I2 have insufiicient current-carrying capacity to pass the peak current for long, the mechanical switch S7 is closed shortly (about 2 milliseconds) after 12 fires, at time t4. The peak current is now maintained for the time taken by the delay line to complete its discharge, at which time, 15, I4 is fired and the current and voltage die away exponentially.

Because 56 and S7 take a relatively long time to operate (about 40 and 57 milliseconds respectively), the discharge cycle is actually started, at time t6, by initiating the closure of S7, the closure of S6 being initiate-d shortly after at time 17. The closure of these switches is controlled by a timing unit 6 which is controlled in turn by a master timer 7 which operates the charging switches S8 and S9. An electromechanical interlock ensures that 56 has closed and that $7 is closing before 11 is fired. I2 is fired by a control circuit 8 which is tripped from a potentiometer R11 connected across C6. 14 is fired by a similar control circuit 9 tripped from a potentiometer R12 connected across the delay line adjacent to switch S6. To prevent the possibility of the voltage on capacitors C1-C5 reversing when the voltage front is reflected from the far end of the line, e.g. due to a short circuit at the S6 end, ignitron I3 is connected across the far end and h fired by a control circuit 10 tripped from a potentiometer R13 connected across the line.

The repetition period of the discharges is variable between 50 and seconds. Since this is a relatively long period, it is preferred to keep the reactor conditioned between discharges by a series of smaller discharges at a higher repetition frequency, and for this reason the transformer T1 is provided with a further primary winding L20 connected to a lower energy energising circuit 12 substantially the same as that described in the aforementioned specification. The repetition period of circuit 12 is controlled by the timer 7 to be about 10 seconds, as in the reactor described in said specification.

The maintaining voltage developed across the secondary windings L6, L16, L17, L18 and L19 can be varied by adjusting the tappings on transformer T2 and/ or the voltage to which capacitors C1-C5 are charged by source 18. In order to match the impedance of the delay line to the resistive impedance of the discharge, after the latter impedance has been transformed through T1 and T2, the values of the inductors L1L4 can be adjusted by means of tappings thereon. In this way the impedance of the line can be varied between 1.8 and 14.35 ohms. This of course also varies the delay line discharge time, i.e. the time between 12 and t5, between a minimum of 30 milliseconds and a maximum of 230 milliseconds. It is also ossible to adjust the impedance by altering the values of capacitors C1-C5, each of which consists of a plurality of smaller capacitors connected in parallel. With a 6:1 ratio on T1, discharge impedances in the range 7.7x 10 to 117x101 ohms can be matched by adjusting inductors Ll-L4, taking into account the range of step-down ratios on T2. With a 6:1 ratio on T1, the pulse rise time, i.e. 12-13, is about 1.8 milliseconds; the 12:1 and 24:1 ratios give rise times of 3.6 and 7.2 milliseconds respectively.

Capacitor C7 is included in the circuit so that if one of the five ignitrons I1 fires slightly before the others, the voltage across the remaining four will not fall immediately and prevent them firing. Capacitor C7, together with the inductance of the lead between it and the transformer primary winding L8, impose a short delay on a pulse arriving at the anode of I1 from one of the other four circuits, and so give it time to fire.

To reduce the circuit inductance, coaxial cables are used for connections carrying the current pulses.

It will be appreciated that the above described embodiment is only one arrangement according to the invention, and that there are other ways in which the invention can be carried out. For example the reason for using five primary windings is that the current-carrying capacity of ignitrons is at present limited. With a discharge current of 10 a., the total primary current with a 6:1 turns ratio is ka., which is much more than a single ignitron could handle. By using five primary windings, and five ignitrons, the current in each is reduced to about 35 ka.

Another possible modification is to use separate primary windings for building up the discharge (period r2-t3) and for maintaining it (period t4-t5). In fact one merit of circuits according to the invention is that the two functions of (a) building-up the discharge current and (b) maintaining it while reactions take place, are separated. The first function requires a short timeconstant circuit and a high electric field to build up rapidly the magnetic energy associated with the discharge channel; this current is determined largely by the circuit inductance and relatively little energy is dissipated. In the second condition the energy to be supplied is determined by the losses from the discharge (radiation, conduction to the torus walls, etc); the current is resistive and only requires a small electric field which must, however, be maintained for a relatively long time.

In this specification the term switch means includes both mechanical switches and discharge devices such as ignitrons.

Iclaim:

1. Gas discharge apparatus wherein a high-current ring discharge is induced in gas in a vessel by the application of a short high-voltage pulse from a first energising circuit to a primary winding of a pulse transformer whereof said ring discharge forms a single turn secondary winding characterised in that there is provided a second energising circuit adapted to produce a relatively low voltage output of relatively long duration arranged to be coupled to said secondary winding in order to maintain the discharge current at a high value for a prolonged period when the short highwoltage pulse has fallen to a low value, and in that said first and second energising circuits are connected in series across the same primary winding, said low voltage output being made available in the series circuit prior to the commencement of said short high voltage pulse.

2. Apparatus as claimed in claim 1 wherein said second energising circuit comprises a delay line adapted to store energy at a high voltage and a step-down pulse transformer for coupling said delay line to said primary winding and hence to said secondary winding.

References Cited in the file of this patent UNITED STATES PATENTS Parson-s Feb. 5, 1957 Kruskal et a1 Jan. 4, 1962 OTHER REFERENCES Nature, Jan. 25, 1958, pages 226-228. NYO-7899, Controlled Thermonuclear Processes, United States Atomic Energy Commission, The Proposed Model C Stellerator, August 29, 1957, pp. 16, 17, 26, 230, 239, 263-273, 304-308. 

1. GAS DISCHARGE APPARATUS WHEREIN A HIGH-CURRENT RING DISCHARGE IS INDUCED IN GAS IN A VESSEL BY THE APPLICATION OF A SHORT HIGH-VOLTAGE PULSE FROM A FIRST ENERGISING CIRCUIT TO A PRIMARY WINDING OF A PULSE TRANSFORMER WHEREOF SAID RING DISCHARGE FORMS A SINGLE TURN SECONDARY WINDING CHARACTERISED IN THAT THERE IS PROVIDED A SECOND ENERGISING CIRCUIT ADAPTED TO PRODUCE A RELATIVELY LOW VOLTAGE OUTPUT OF RELATIVELY LONG DURATION ARRANGED TO BE COUPLED TO SAID SECONDARY WINDING IN ORDER TO MAINTAIN THE DISCHARGE CURRENT AT A HIGH VALUE FOR A PROLONGED PERIOD WHEN THE SHORT HIGH-VOLTAGE PULSE HAS FALLEN TO A LOW VALUE, AND IN THAT SAID FIRST AND SECOND ENERGISING CIRCUITS ARE CONNECTED IN SERIES ACROSS THE SAME PRIMARY WINDING, SAID LOW VOLTAGE OUTPUT BEING MADE AVAILABLE IN THE SERIES CIRCUIT PRIOR TO THE COMMENCEMENT OF SAID SHORT HIGH VOLTAGE PULSE. 